Flexible graphite has been successfully used as a packing material in fluid handling devices, such as pumps and valves, to provide a seal where a reciprocating stem or shaft enters the device. Commonly the flexible graphite is in the form of preformed annular rings which are placed around the stem in a stuffing box and compressed. For example, U.S. Pat. No. 4,190,257 discloses packing rings of flexible graphite particles compressed together into annular seal rings. Packing seal rings of flexible graphite may also be formed by wrapping a flexible graphite tape about a shaft to form an annular shaped article and compressing the article in a die.
Flexible graphite seal rings, since they are essentially pure carbon, are chemically inert and stable at high temperatures. In addition, the resilience of the flexible graphite allows for very effective seals. Because of these qualities, flexible graphite is a preferred packing material in high temperature and corrosive environments.
However, packings with flexible graphite seal rings are prone to produce "chatter" when used as seals for reciprocating valve stems, Particularly at temperatures less than about 1100.degree. F. (600.degree. C.). Chatter is an erratic slip-stick motion of the stem as it passes through the packing. It is a common phenomenon. Particularly in applications where little force is being applied to the stem to move it through the packing or where precise control is required. Such applications include pneumatically controlled valves wherein a diaphragm or piston actuates a reciprocating stem. Because of chatter, precise control of such valves is difficult or impossible.
Chatter can be reduced by applying a lubricious material on the surface of the stem, such as water. Petroleum derivatives, or the like. However, these materials generally provide only a temporary solution as they often volatilize or degrade, particularly when the valve is subject to high temperatures. The loss of the lubricious material then results in the return of the chatter of the stem.